1. Field of the Invention
The present invention relates to a light receiving circuit for detecting a change in amount of light, and more particularly, to a light receiving circuit capable of stable detection of a change in amount of light regardless of ambient light conditions.
2. Description of the Related Art
Light receiving circuits are used for receiving optical signals in infrared remote control communications or visible light communications and used for photointerrupters, distance sensors, and the like. The light receiving circuit needs to function to detect an abrupt change in amount of light generated when an LED or the like is turned on without detecting a slow change in amount of light generated when a person moves or an object swings in the wind or a change in amount of light of a fluorescent lamp whose brightness fluctuates with a period of 50 Hz. It is also important for the light receiving circuit to maintain light receiving sensitivity regardless of ambient light conditions.
FIG. 6 illustrates a block diagram of a related-art light receiving circuit. The related-art light receiving circuit includes a photodiode 101, a resistive element 601, a low pass filter 603, and an NMOS transistor 602.
The photodiode 101 has an N-type terminal connected to a VDD terminal, and a P-type terminal connected to an output terminal 604 and one electrode of the resistive element 601. The other electrode of the resistive element 601 is connected to a GND terminal. The low pass filter 603 has an input terminal 610 connected to the one electrode of the resistive element 601 and an output terminal 611 connected to a gate of the NMOS transistor 602. The NMOS transistor 602 has a drain connected to the one electrode of the resistive element 601 and a source connected to the GND terminal. The output terminal 604 is connected to the one electrode of the resistive element 601.
The light receiving circuit having the above-mentioned configuration operates as follows to detect a change in amount of incident light.
When the environment is dark, no steady current flows through the photodiode 101, and hence a voltage at the output terminal 604 becomes a GND terminal voltage and the NMOS transistor 602 is off. When the photodiode 101 is irradiated with light of an LED or the like, a current is generated in the photodiode 101. This current flows through the resistive element 601 to generate a voltage. When this voltage is output to the output terminal 604, it can be detected that the amount of incident light has changed.
When the environment is bright, a steady current flows through the photodiode 101. The voltage of the output terminal 604 increases because the current flows through the resistive element 601. When the voltage of the output terminal 604 exceeds a threshold voltage of the NMOS transistor 602, the NMOS transistor 602 is turned on. Thus, the voltage of the output terminal 604 is controlled to be around the threshold voltage of the NMOS transistor 602. In other words, no matter how bright the environment is, the voltage of the output terminal 604 increases at most to around the threshold voltage of the NMOS transistor 602. When the photodiode 101 is irradiated with light of an LED or the like, the current of the photodiode 101 increases. In this case, because a gate voltage of the NMOS transistor 602 changes via the low pass filter 603, the instantaneously-changing current flows only through the resistive element 601. Thus, this current increases the voltage of the resistive element 601, thereby increasing the voltage of the output terminal 604. Then, when the voltage of the output terminal 604 becomes equal to or higher than a predetermined voltage, it can be detected that the amount of incident light has changed.
As described above, the related-art light receiving circuit includes the low pass filter 603 and the NMOS transistor 602, and consequently the light receiving sensitivity is not affected by the ambient light conditions (see, for example, Japanese Patent Application Laid-open No. Hei 09-083452).
In the related-art light receiving circuit, however, a large parasitic capacitance of the photodiode, a wiring capacitance, and the like are present at the node between the photodiode and the resistor, thus decreasing a voltage increase rate at the node whose voltage is increased by the current of the photodiode. Thus, when an LED or the like is turned on at a distant location, a generated current of the photodiode is small and the voltage increase rate at the node is slow, and hence the NMOS transistor causes the current of the photodiode to flow before the voltage of the node reaches a predetermined value. In other words, the related-art light receiving circuit has a problem of low sensitivity.